In a known stack forming device a traction surface is provided on an outer surface of a roller, which is rotatably driven for moving a sheet into a sheet retaining position. The trailing edge of the sheet is kept in contact with the traction surface during a rotation of the traction surface by providing friction between the sheet and the traction surface, for example by applying a vacuum pressure between the sheet and the traction surface. At some point the trailing edge of the sheet is released and the sheet is propelled into a sheet retaining position.
The known stacking device has the disadvantage that the traction surface provides a limited control over the movement of the sheet into a sheet retaining position during the rotation of the traction surface. Another disadvantage is that the stacking device provides insufficient control of stacking a plurality of sheets, in case the plurality of sheets has a substantial variation in length.
In another stack forming device a rotatable guide structure having paddle arms is used for moving a sheet into a sheet retaining position. The rotation of the paddle arms is controlled so that first the paddle arms support a trailing edge of a sheet during stacker input of a sheet. Thereafter the paddle arms are rotated 360° to clear the trailing edge of the sheet from the stacker input path. Thereby the sheet trailing edge is released towards the top of the sheet stack.
The alternative stack forming device has the disadvantage that the paddle arms provide a limited control over the movement of the sheet into a sheet retaining position. Another disadvantage is that the stacking device provides insufficient control of stacking a plurality of sheets, in case the plurality of sheets has a substantial variation in length.